The present invention relates to imaging apparatus such as a digital camera, digital video camera, etc. using a solid-state imaging device.
In recent years, MOS solid-state imaging devices capable of forming peripheral circuits on chip have been remarkably improved in their performance and are being spread. With a MOS solid-state imaging device, a plurality of transistors such as an amplification transistor and a reset transistor in addition to a photoelectric conversion device are provided in each one pixel. Variances in threshold of the transistors among pixels and kTC noise (thermal noise) at the time of their resetting are the cause of a fixed pattern noise and/or random noise in image. To remove these noises, a differential processing is effected between a reset level immediately after the resetting and a signal level. In those solid-state imaging devices where such a differential processing is effected, a problem referred to as black-sun phenomenon occurs.
FIG. 1A shows a dependency of reset level and signal level on light amount. The vertical axis indicating signal level in this case is shown so that “+” direction indicates the direction of change by incidence of light. As shown in FIG. 1A, a signal Vs outputted from the amplification transistor to which signal after photoelectric conversion by photoelectric conversion device is inputted contains a signal generated by the photoelectric conversion and a reset signal Vr. From the solid-state imaging device, therefore, a difference signal (Vs−Vr) is outputted as signal generated by the photoelectric conversion by subtracting the reset signal Vr from the signal Vs (differential processing).
FIG. 1B shows a dependency on light amount of the difference signal (Vs−Vr). Since the signal Vs is saturated (level of Vssat in FIG. 1A) when the incident light amount is more intense than point B, the difference signal (Vs−Vr) is also saturated to attain a certain value (level of Vsat in FIG. 1B). Although the reset signal Vr maintains a certain level (level of Vr1 in FIG. 1A) even when the signal Vs is saturated, the level of the reset signal Vr is changed as shown in FIG. 1A when an even stronger light that is more intense than point C is incident. For this reason, the difference signal (Vs−Vr) becomes smaller as shown in FIG. 1B to result in the black-sun phenomenon.
The above described level change of the reset signal Vr occurs due to the fact that, because of light leak, a light leak noise signal is added to the reset signal at an input section of the amplification transistor when an extremely intense light is incident on the photoelectric conversion section. This is the condition of regions where the incident light amount is more intense than point C, and the difference signal (Vs−Vr), i.e. the result of the differential processing is reduced. When the reset signal Vr reaches its saturation (point E) due to the light leak noise signal, the difference signal (Vs−Vr) becomes 0 and the black-sun phenomenon occurs.
If such black-sun phenomenon occurs, for example in the case of photographing the sun, an unnatural image results as a center portion of the sun becomes a black spot as shown in FIG. 2A. Shown in FIG. 2B is a conceptual drawing where the sun and its periphery within the dotted line frame in FIG. 2A are shown in correspondence to the light amounts in FIG. 1B. A to E in FIG. 2B represent the light amount levels A to E in FIG. 1B. In this manner, since the light amount in periphery of an intense incident light is generally gradually reduced, it as an image results in something like the pattern of a donut where the levels change about the region on which the intense light is incident.
As a method to suppress such black-sun phenomenon, Japanese Patent Application Laid-Open 2000-287131 discloses one in which a change in the output of reset level is detected and, when it is determined as an occurrence of the black-sun phenomenon, a predetermined value is written as the reset level output. Further, Japanese Patent Application Laid-Open 2007-20156 discloses another in which a clamp circuit is provided on a signal line to which pixel outputs are commonly connected so as not to allow a change exceeding a predetermined level in the output of the reset level.
Basically in both of the prior-art techniques disclosed in the above publications, a change in the reset level output is detected and, when it is determined as an occurrence of black-sun phenomenon, the reset level or the signal level or the result of differential processing (difference signal) is set to a predetermined value so as to correct a final output of the pixel where the black-sun phenomenon occurs. In detecting the change in reset level output, since there are variances in pixel and the circuit for detecting output change, a detection level must be set with a margin. Accordingly, there has been a problem that a dynamic range of the final output is reduced corresponding to such margin.
The manner of occurrence of this problem will be described below by way of FIGS. 1A and 1C. It is supposed in FIG. 1A that a level indicated by Vref is the detection level for detecting a change in the output of reset level. This detection level Vref is set with a certain margin considering the above described variances. In the case of the illustrated example, an occurrence of black-sun phenomenon is determined when the light amount exceeds the point of D, and it is corrected for example so that the result of differential processing (difference signal) attains a level that securely exceeds that of saturation. The dependency on light amount of the differential processing result (Vs−Vr) thereby attains a characteristic as shown in FIG. 1C so that an occurrence of the black-sun phenomenon is unavoidable in the portion where the light amount is from point C to point D. For this reason, the differential processing result (Vs−Vr) is clipped at a level indicated by Vb in FIG. 1C through a processing circuit at a later stage. Accordingly, the dynamic range is reduced in relation to Vsat that is a theoretical saturation level.